This invention relates to a control for a solid state adjustable speed induction machine drive and more particularly to a scalar decoupled control for an induction machine supplied by a current-controlled voltage-fed inverter for achieving fast transient response.
In many applications, such as servo and rolling mill drives, the drive system is required to have fast transient response and the capability to operate at zero speed with full torque. Traditionally, for such applications, dc machines have been used. The dc machines are characterized by inherent decoupling between the armature or torque component of current and field flux, and therefore, have a fast response characteristic to changes in command torque. The ac machines, particularly the cage type induction motors, are attractive in industrial applications because of the absence of commutators and brushes and low rotor inertia. The induction machine is a complex multivariable nonlinear coupled system where each of the outputs is a function of the input variables. Because of the coupling between input and output, the conventional scalar or dc signal control with torque and flux feedback loops fails to give adequate transient response to torque command changes. If, for example, a step torque demand is established by incrementing the slip signal with the desired rated flux, the flux will diminish temporarily until compensated by a feedback loop in a sluggish manner.
The field-oriented or vector control theories have been advanced in the recent years to solve the coupling problem in ac machines. The theories are based on the synchronously rotating direct axis and quadrature axis (d-q) model of the machine in which variables appear as dc quantities in the steady-state condition. The d and q component of stator current in this model are mutually decoupled and can be controlled as a flux component and a torque component of current, respectively. To convert the d and q components of command signals from the rotating to the stationary reference frame, feedback of unit electrical vectors in appropriate phase position is necessary. In Blaschke's or direct method of vector control, the unit vectors are obtained from rotor flux which can either be based on measurement or computation. In Hasse's or indirect method of vector control, the unit vectors are synthesized by addition of the rotor mechanical position vector and the commanded slip angle vector derived from the torque component of current. Both vector control methods require complex coordinate transformation, phase conversion, and intricate vector signal sensing and processing. In the former method, a flux coil may be considered undesirable; and the control based on computation may be difficult to implement due to harmonics, and practically impossible to implement near zero speed. Although the latter method can be implemented satisfactorily down to zero speed and is somewhat free from feedback harmonics, a precision position encoder on the machine shaft may not always be desirable. While it is desirable to control ac machines in a decoupled manner as dc machines, thereby achieving dc machine-like transient response, it is difficult in practice to eliminate coupling. For example, during current transients when the feedback current does not match the commanded current, the vector diagram on which the theory is based does not hold true, and partial coupling dictated by the d-q machine transients will exist. Again, both control methods are somewhat dependent on machine parameters, the inaccuracy and unpredictable nature of which worsen the coupling effect.
It is an object of the present invention to provide an induction motor control that achieves fast response at various operating points using a simplified control.
It is a further object of the present invention to achieve decoupling between the commanded frequency and rotor flux in a scalar control during static and dynamic conditions at all operating points.
It is a still further object of the present invention to provide a scalar decoupled control having fast transient response and four-quadrant operating capability.